1. Field of the Invention
The present invention pertains to apparatus for positioning an object precisely with at least four and as many as six degrees of freedom, and in particular to a positioning stage for use in masked ion-beam lithography.
2. Description of the Related Art
The design and production of very large scale integrated (VLSI) circuitry components requires an assortment of costly apparatus and sophisticated processing techniques. Lithography is the process bringing together the many techniques for selectively removing or adding material to the semiconductor wafers from which the circuit chips are ultimately fabricated. One of the most promising techniques in this area of technology is masked ion-beam lithography, in which a collimated beam of ions passes through a mask onto a semiconductor wafer covered with photoresistive material. The advantage of using ion-beam lithography is that it allows extremely high pattern resolution. The massive ions have a relatively short mean free path in the photoresist material, and the secondary electrons produced in collisions have relatively low energy and also do not travel very far. Because there is some ion scattering when the ions travel through the mask, the mask and wafer must be positioned very closely to each other (approximately 25 micrometers) to achieve high-resolution exposures.
Once the mask is fixed in location, alignment of the wafer to the mask requires precise motions with at least four and as many as six degrees of freedom. The movements need to be accomplished very rapidly, and the wafer needs to be held rigidly in place once it is precisely located.
Hansen U.S. Pat. No. 4,528,490, assigned to Hughes Aircraft Company, the assignee of the present invention, discloses a two-axis drive for a positioning stage. The drive includes, for each stage, a drive bar frictionally engaged against a drive capstan and held in place by a floating pressure roller so that the pressure roller can swing as the drive bar swings.
Reeds U.S. Pat. No. 4,532,426, assigned to Hughes Aircraft Company, discloses a wafer height correction system for a focused beam system. The base plate of the wafer support is flexibly mounted with respect to the floor of the target chamber. Metal diaphragms flex by operation of one or more motors to adjust the position of the wafer support with respect to the focal point of the column.
Typical positioning apparatuses used for wafer lithography incorporate a rotating stage on top of a translational stage. Translational motion of the wafer in a plane and rotation of the wafer about an axis normal to that plane are allowed. The plane of translational motion is commonly referred to as the X-Y plane, and the angle of rotation in the X-Y plane as .theta.. With this sort of arrangement, the mass of the rotating element must be moved in changing the X and Y positions of the wafer. The added inertia of the rotating stage reduces the speed of response that is obtainable. In addition, with this sort of arrangement, as the X and Y positions are varied, the center of rotation for .theta. moves relative to the system axis. In registering the lithography mask to the wafer, if the center of rotation does not lie on the system beam axis, then each chip on the wafer will require a different algorithm to use the mask-mask alignment sensing measurements to compute the necessary rotational corrections to align the mask and the wafer. Also, in conventional positioning systems the wafer is not fixed relative to the interferometer mirrors used in determining the position of the wafer. This makes the procedure of measuring the X-Y position of the wafer more complicated than it would be if the position of the wafer were fixed relative to the interferometer mirrors.
In conventional direct-write systems using a focused electron or ion beam to create the patterns on a chip, the center of rotation for the positioning stage does not coincide with the beam axis The pattern to be written on the chip is programmed in X-Y coordinates on a computer which controls the scanning of the beam. Any rotational misalignment that is compensated for by rotating the wafer creates an X-Y shift of each chip that varies with the position of the chip on the wafer. The X-Y coordinates must now be transformed for each chip to take account of the rotation.